In the art thin-layer electroluminescent devices are known which are actuated alternating current (AC-TFEL: alternating current-thin film electroluminescence), used in variable-information displays and in image displays.
For example, L. E. Tannas, in "Flat Panel Displays and CRT's", Van Nostrand-Reinhold, New York, 1985, page 237, describes the use of AC-TFEL devices in flat displays.
The AC-TFEL devices known from the prior art are generally provided with a structure formed by a glass support, on which there are deposited, in succession: a metal layer, an insulating layer, a luminescent layer, an insulating layer, and a transparent conductive layer.
In particular, the metal layer can be formed by aluminum or gold; the insulating layers can be formed by yttrium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), barium titanate (BaTiO.sub.3), silicon nitride (Si.sub.3 N.sub.4), or their mixtures; the luminescent layer can be formed by zinc sulphide (ZnS) and zinc selenide (ZnSe), with luminescent impurities of manganese; and the transparent conductive layer can be formed by indium oxide (In.sub.2 O.sub.3), tin oxide (SnO.sub.2) or relevant mixtures, e.g., a mixture containing indium oxide and tin oxide in mutual proportions of the order of 9:1 by weight.
As reported by D. Theis et al., in J. Crystal Growth 63, 47 (1983) and by D. Theis, in Physica Status Solidi (a) 81, 647 (1984), the electro-optical characteristics of the thin-film electroluminescent devices mainly depend on the microstructure of the luminescent layer.
More particularly, the luminescent layers prepared by means of the usual flash evaporation technique and sputtering technique, typically show a microcrystalline structure, with grains having side dimensions smaller than 0.050-1.0 .mu.m, for thicknesses of up to approximately 0.2 .mu.m. When the thickness reaches the value of approximately 0.5 .mu.m, the side dimension of the grains increases up to approximately 0.3-0.5 .mu.m. With such a microstructure of the luminescent layer, peak voltages generally higher than 150 V are required in order to induce electroluminescence in the relevant electroluminescent device.
Furthermore, by using the deposition method known as "Atomic Layer Epitaxy", described, by T. Suntola and J. Hyvarien, in Annual Review of Material Science, 15, 177 (1985), preparing luminescent layers of multicrystal zinc sulphide doped with manganese, good-quality columnar grains, is possible. These grains have side dimensions larger than 0.1 .mu.m and smaller than 1 .mu.m, which remain constant with varying thickness, at least for thicknesses higher than 0.05 .mu.m. With such a microstructure of the luminescent layer, peak voltages lower than a150 V, but higher than 100 V are necessary in order to induce electroluminescence in the relevant electroluminescent device.
In order to further improve the microstructure of the luminescent layer, and therefore the performance of the electroluminescent devices which incorporate such a layer, structures have been proposed in the art, which are based on semiconductor supports, mostly obtained by means of complex and expensive deposition methods. For example, K. Hirabayashi and K. Katoh, in the Japanese Journal of Applied Physics, 24, L 629 (1985), describe a structure formed by single-crystal silicon; single-crystal zinc sulphide doped with manganese; insulating layer; and transparent conductive layer; in which the deposition of the active layer of zinc sulphide is carried out by means of molecular beams and wherein the single-crystal structure of the layer is obtained by epitaxy on single-crystal silicon (Molecular Beam Epitaxy). By means of such a device, the peak voltages, suitable for inducing electroluminescence, can be lower than 100 V.
On the basis of such a present state of the prior art, the purpose of the present invention is an electroluminescent device having good electro-optical characteristics, and a low threshold voltage for electroluminescence, which can be obtained by means of the usual flash evaporation technique or sputtering technique.